R编程：数据帧到矩阵的转换

Question

I am trying to populate a matrix from a dataframe in R.

# A demo Data Frame
d = data.frame(movie=c(1,2,3),user=c(1,3,2),rating=c(1,4,2))
# Initialize Output Matrix
output<-matrix(data=NA,nrow = length(d$movie),ncol = length(d$user),byrow = FALSE,)


#Function 
# x= movie, y = user, input=data frame to be indexed, out = output matrix
getMatrixFilled<-function(x,y,input,out){
      out[x,y]<-input[x,y];
      out
    }

I want to take movie , user and the corresponding rating and place it matrix

as

 output[movie,user]<-rating

Thanks for help in advance!!

Input

> d
  movie user rating
1     1    1      1
2     2    3      4
3     3    2      2

Desired Output

> outputM
     [,1] [,2] [,3]
[1,]    1   NA   NA
[2,]   NA   NA    4
[3,]   NA    2   NA

Answer 1

You could try

output[as.matrix(d[1:2])] <- d$rating
output
#     [,1] [,2] [,3]
#[1,]    1   NA   NA
#[2,]   NA   NA    4
#[3,]   NA    2   NA

Answer 2

I suspect you want a simple reshape:

library(reshape2)
acast(d, movie ~ user)
#   1  2  3
#1  1 NA NA
#2 NA NA  4
#3 NA  2 NA

Answer 3

Since we're guessing on the desired output, here are a couple of other alternatives that work similarly to the existing answers:

xtabs(rating ~ movie + user, d)
#      user
# movie 1 2 3
#     1 1 0 0
#     2 0 0 4
#     3 0 2 0

library(tidyr)
spread(d, user, rating)
#   movie  1  2  3
# 1     1  1 NA NA
# 2     2 NA NA  4
# 3     3 NA  2 NA

And (by popular demand), base R's reshape (but the columns are sorted differently)...

reshape(d, direction = "wide", idvar = "movie", timevar = "user")
#   movie rating.1 rating.3 rating.2
# 1     1        1       NA       NA
# 2     2       NA        4       NA
# 3     3       NA       NA        2

Answer 4

I think @akrun showed you the simplest way. (You were almost there. but the indices needed to be delivered to "[" in the form of a matrix.) I'm posting the application of using a two-column matrix for indexing assignment into matrix locations to the format of the function you requested.

 d = data.frame(movie=c(1,2,3),user=c(1,3,2),rating=c(1,4,2))

 output<-matrix(data=NA,nrow = length(d$movie),ncol = length(d$user) )
 getMatrixFilled<-function(x,y,input,out){
       out[cbind(x,y)] <- input[ , "rating"]
       out
     }
 getMatrixFilled(x=d$movie, y=d$user , input=d, out=output)
#--------------
     [,1] [,2] [,3]
[1,]    1   NA   NA
[2,]   NA   NA    4
[3,]   NA    2   NA